Revealing mistake: When Steve Christy is leaving the diner, it shows the outside of the diner, where it is raining. It's very obvious that the "rain" is just a sprinkler.
Revealing mistake: When Brenda's body is thrown through the window, the movement of her head when hitting the floor is not "limp". She positions her head for the landing, then deliberately tilts it to the side as if dead. Also, on the second cut from Alice to Brenda we can see Brenda's head moving ever so slightly. (01:14:17)